1. Field of the Invention
The present invention relates to a process for preparing olefinically unsaturated compounds such as, in particular, styrene by oxidation or oxidative dehydrogenation in the gas phase by means of redox catalysts in the fluidized bed.
Styrene monomer (SM) is an important monomer for engineering plastics and is used in large amounts. It is prepared virtually exclusively by (nonoxidative) dehydrogenation of ethylbenzene (EB) at about 600.degree. C. The dehydrogenation is an equilibrium reaction which is carried out on an industrial scale so that a conversion of about 60-70% is achieved and unconverted EB is thereafter separated off and recycled. The reaction is endothermic.
Complete conversion is only possible with processes which permit removal of the hydrogen from the reaction mixture. Oxidative dehydrogenation, which is exothermic, has therefore been suggested as a way of overcoming the equilibrium.
2. Description of Related Art
In conventional (direct) oxidative dehydrogenation, oxygen is fed together with the reaction mixture and passed over a single catalyst, generally of the fixed type. Water forms and is removed from the reaction equilibrium, so that virtually quantitative conversion is achieved at a relatively low temperature. Since oxygen is usually used in excess, the deposition of byproducts (coking) on the catalyst is not overly critical and it is possible in this way to maintain a steady state process over long periods and achieve a high space-time yield. A disadvantage of oxidative dehydrogenation is the occurrence of hitherto unavoidable side reactions which lead to total oxidation and hence loss of product of value.
To avoid this disadvantage, EP-A-0 336 622 proposes employing a plurality of catalyst systems by providing initially two or more dehydrogenation zones and feeding the oxygen-containing gas in at a plurality of points and then passing the product stream over an oxidation catalyst. For the dehydrogenation of ethylbenzene it is suggested that a conventional iron oxide catalyst be coupled with a downstream noble metal oxidation catalyst.
Another way of avoiding the direct contact of the reactants with free oxygen is based on the separation of the elementary steps of the reaction in space or time by means of a redox catalyst acting as an oxygen store and transfer agent. This process is known as an unsteady state or indirect process and has been proposed for various chemical reactions. Examples are the unsteady state oxidation and ammonoxidation of propene, the oxidative dehydrogenation of alkanes and alcohols, the oxidative dehydrogenation of mono- to diolefins, the oxidative coupling of methane to form higher hydrocarbons, the dehydrodimerization of toluene to stilbene, the dehydrocyclization and dehydroaromatization of paraffin hydrocarbons, the oxidation of butadiene and the oxidation of butane. Bi- and V-containing redox catalysts are mentioned as possible catalysts.
The redox catalyst catalyzes the hydrocarbon oxidation reactions by giving off lattice oxygen to form water and is reduced at the same time. The reduced catalyst is subsequently reoxidized by molecular oxygen. During the regeneration step any coke deposits on the catalyst are removed as well, so that the original activity is generally completely restored. The cycle is constantly repeated.
The unsteady state procedure is implemented in industrial practice in processes for waste gas cleanup, SCR removal of nitrogen oxides and in sulfuric acid production. Spatial separation on an industrial scale is also practiced in the case of cat crackers, where the cracking reaction and the subsequent regeneration of the catalyst to burn off coke deposits are separated in space via circulating fluidized bed reactors. Catalytic reforming (isomerization of hydrocarbons in the refinery art) is carried out using a migrating bed. It has also been proposed to employ spatial separation in a riser-regenerator fluidized bed for the oxidation of butane to maleic anhydride, and an unsteady state process is also used in industry for the dehydrogenation of propane (Catofin process). Four separate, adiabatic fixed bed reactors are used, which successively pass through the operating modes of dehydrogenation--rinsing--regeneration--rinsing.
According to EP-A-039 737 and EP-A-403 462, the principle of unsteady state reaction management can be used for the oxidative dehydrogenation of ethylbenzene to styrene, and numerous redox-active elements are named as useful catalyst components. Preference is given to V/MgO. The unsteady state dehydrogenation is also described in U.S. Pat. No. 4,067,924 with Mg-chromite catalysts and in U.S. Pat. No. 3,842,132 with Bi--Cr vanadates.
According to a proposal unpublished at the priority date of the present invention, Bi- and V-containing catalyst systems can be used for the oxidative dehydrogenation of ethylbenzene to styrene. Preference is given to the K/Cs/La/Bi/TiO.sub.2 catalyst.
In unsteady state operation, conversion and selectivity are not constant over the operating cycle. At the start the catalyst is, say, in the oxidized state and is highly active. The reaction rate is correspondingly high, which also entails a certain increase in the byproduct level (gasification to carbon oxides, for example) associated with an arithmetically lower selectivity. As the degree of reduction of the catalyst increases, byproduct formation decreases and selectivity improves continuously to an end value specific to the particular catalyst employed. On the other hand, the catalyst becomes more and more deactivated at the rate of consumption of its lattice oxygen, so that the conversion decreases and the catalyst finally has to be regenerated. The net result is that the styrene yield, being the product of selectivity and conversion, generally passes through a flat maximum.
In industrial practice, the catalyst will not be used until it is completely deactivated; instead, regeneration will be initiated while conversion is still economically acceptable. Because the catalyst was only partially reduced, the regenerating time can be shorter, too.
Partial prereduction with H2 or CO has been proposed as a remedy against initial gasification (EP-A-482 276, JA-A-133 602, JA-A-127 819).
An advantage of unsteady state oxidation and oxydehydrogenation over direct oxidation is in any event the selectivity gain through a reduction in total combustion, since reactants and oxygen are no longer present in the reaction mixture at one and the same time. There are also advantages in the workup ("integrated separation process"). A disadvantage, on the other hand, is the relatively low space-time yield, since no product of value is produced during the regenerating period and in the rinsing periods. In the unsteady state oxydehydrogenation of ethylbenzene to styrene the cycle would typically have to consist of 15 minutes each of dehydrogenation and regeneration and two rinsing periods of 5 minutes each, resulting in a productive period of 15 minutes and waiting times of 25 minutes in total. This would require large catalyst masses and correspondingly large reactors.